Woooow, this is GREAT. Yeah I very much get the Half Life feel, as Autumn Spooks mentioned. It actually reminds me of the soundtrack for the remake, Black Mesa, composed by Joel Nielsen; similar vibe for sure.
I think one of my main points of feedback would be it's the drums (particularly the snare ) sound very forward in the mix. I think maybe bringing them back at bit - e.g compressing the attack on the snare - to let some other instruments breathe might work? Particularly from 4:30 onward where there's some beautiful post-rock guitar.
Otherwise I loved it! That short piano motif throughout really anchors the piece :-D I totally get the space-like feel, would be entirely out of place in a game like Destiny.
Great job!!